Thermoplastic resin films, above all, biaxially oriented polyester films are widely used as substrate films for many applications such as optical members, peripheral materials of displays, display members, packaging materials, electric insulation materials, various photosensitive materials and graphic art materials, since they are excellent in such properties as dimensional stability, mechanical properties, heat resistance, transparency and electric properties.
In general, since biaxially oriented polyester films are highly crystal-oriented on the surfaces, they have had a disadvantage of being poor in the adhesive properties to various paints and inks. Therefore, various methods have been studied to provide the polyester film surfaces with adhesive properties.
Known methods of imparting adhesive properties include, for example, surface activation methods such as corona discharge treatment of substrate polyester film surfaces, surface etching methods using such chemicals as acids and alkalis, and methods of forming primer layers of various resins such as acrylic resins, polyester resins and urethane resins on film surfaces. Among them, especially as methods of imparting adhesive properties by coating to form the above-mentioned primer layers, methods comprising the steps of coating a polyester film not yet completed in crystal orientation with a coating liquid containing any of the above-mentioned resins as an ingredient, drying, subsequently stretching and heat-treating to complete the crystal orientation (inline coating methods) are considered to be promising in view of process simplification and production cost, and are popularly employed in the industry concerned.
For example, a polyester film coated with a combination consisting of two polyester resins different from each other in glass transition temperature and a crosslinking agent such as melamine or polyisocyanate (JP 10-86303 A and JP 60-106839 A) and the like are proposed.
These methods are very effective in the case where a hard coat layer, printing layer or adhesive layer, etc. is formed on an organic material such as a polyester film by coating or sticking, etc. However, recently higher values are added to components, and in this connection, lamination between dissimilar materials, for example, lamination between a surface-treated metal layer of iron or the like and an organic material such as a polyester film as typified by can lamination, etc. is practiced. Especially for imparting the adhesive property to a metal layer, it is generally known that the method of keeping the surface of a polyester resin amorphous is effective, and it is also known that baking coating of a melamine-based crosslinking agent or the like allows good adhesion to a metal layer. For example, a polyester film coated with a melamine-based crosslinking agent (JP 11-198329 A and JP 2005-342981 A) and the like can be referred to.
However, the aforementioned prior art has the following problems.
With regard to imparting adhesive properties, for example, a method of forming a primer layer consisting of two polyester resins different from each other in glass transition temperature alone on a film surface and a method of forming a primer layer further additionally containing a melamine-based crosslinking agent instead of the primer layer can provide an excellent adhesive property to printing ink or the like, but such a problem is likely to occur that especially the adhesive property to a metal layer cannot be obtained at all. Further, for example, in the case where the glass transition temperatures of the polyester resins used as described above are low, adhesion failures in the post-processing using the film often occur in relation with not only the adhesive property after a heat and wet test but also chemicals resistance, etc.
On the other hand, for example, in the method of forming a primer layer obtained by adding polyester resins to a large amount of a melamine-based crosslinking agent, if a melamine-based resin layer is formed as the primer layer on a substrate film, releasing function is exhibited and the adhesive property to a metal layer cannot be obtained. In addition, such a peculiar phenomenon occurs that the adhesive property to ordinary printing ink and the like cannot be obtained at all either.
Thus, it could be helpful to overcome the above-mentioned disadvantages by providing a laminated adhesive film having a primer layer containing a melamine-based crosslinking agent together with polyester-based resins, which is excellent in the adhesive property to a metal layer, especially excellent in the adhesive property to a metalized layer, the adhesive property after a heat and wet test and the adhesive property after solvent tests using various chemicals. Further, it could be helpful to provide an ordinary antistatic property and also to significantly inhibit the iridescent colors generated when a hard coat resin or the like is formed on a laminated adhesive film by containing specific carbon nanotubes.